Lorentz microscopy sheds light on the role of dipolar interactions in magnetic hyperthermia

Campanini, Marco; Ciprian, R.; Bedogni, Elena; Mega, A.; Chiesi, Valentina; Casoli, F.; Fernández, César de Julián; Rotunno, Enzo; Rossi, Francesca; Secchi, Andrea; Bigi, Franca; Salviati, G.; Magén, Cesar; Grillo, Vincenzo; Albertini, F.

doi:10.1039/c5nr00273g

Cited by 17 publications

(16 citation statements)

References 47 publications

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“…The phase reconstruction process was performed in STEMCELL software . By taking a second focal series after reversing the orientation of the sample, the electrostatic and magnetic contributions to the phase shift can be separated following a procedure analogous to the time‐reversal approach proposed by Tonomura for off‐axis electron holography. Such separation approach is strictly required to correctly characterize a system displaying a complex shape, e.g., nanodisks, whose geometry is a truncated cone with a local thickness change at the edges, to avoid shape‐related artefact in the phase reconstruction process (Figure S2, Supporting Information).…”

Section: Methodsmentioning

confidence: 99%

Magnetic Shape Memory Turns to Nano: Microstructure Controlled Actuation of Free‐Standing Nanodisks

Campanini

Nasi

Fabbrici

et al. 2018

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Magnetic shape memory materials hold a great promise for next‐generation actuation devices and systems for energy conversion, thanks to the intimate coupling between structure and magnetism in their martensitic phase. Here novel magnetic shape memory free‐standing nanodisks are proposed, proving that the lack of the substrate constrains enables the exploitation of new microstructure‐controlled actuation mechanisms by the combined application of different stimuli–i.e., temperature and magnetic field. The results show that a reversible areal strain (up to 5.5%) can be achieved and tuned in intensity and sign (i.e., areal contraction or expansion) by the application of a magnetic field. The mechanisms at the basis of the actuation are investigated by experiments performed at different length scales and directly visualized by several electron microscopy techniques, including electron holography, showing that thermo/magnetomechanical properties can be optimized by engineering the martensitic microstructure through epitaxial growth and lateral confinement. These findings represent a step forward toward the development of a new class of temperature‐field controlled nanoactuators and smart nanomaterials.

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Section: Methodsmentioning

confidence: 99%

Magnetic Shape Memory Turns to Nano: Microstructure Controlled Actuation of Free‐Standing Nanodisks

Campanini

Nasi

Fabbrici

et al. 2018

Small

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“…[11][12][13][14][15][16] In fact, interactions are the basis of a large number of nanoparticle-based magnetic materials, e.g., superferromagnets, superspin glasses, artificial spin ice, long range self-assemblies, or ferrofluids. 15,[17][18][19][20][21] Given the crucial importance of interactions in magnetic nanostructures, many direct and indirect approaches have been used to try to quantify them: first order reversal curve (FORC) analysis, 22,23 small angle neutron scattering, SANS, [24][25][26][27] electron holography, 28,29 magnetic force microscopy, 30,31 Lorentz microscopy, 32 Brillouin light scattering, 33 resonant magnetic x-ray scattering 34 and so on. However, one of the most accepted methods to assess interactions is the remanence plots technique (i.e., Henkel or δM plots), [35][36][37] which is routinely used to evaluate interactions between nanoparticles or grains [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52]…”

Section: Introductionmentioning

confidence: 99%

Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles

et al. 2017

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Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains.Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the δM plots of γ-Fe 2 O 3 nanoparticles isolated by thick SiO 2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions, as well as offer new perspectives in the use of Henkel-and δM-plots to quantify the rather elusive inhomogeneous magnetizations states in nanoparticles.Additional information on the δM and the in-field Mössbauer techniques, table with the complete results of the Mössbauer spectra fits, details of the Monte Carlo simulations, FC and ZFC magnetization curves of the VST series (Fig. S1a), Langevin scaling of M(H;T) data measured in VST45 (Fig. S1b), details on the estimate of the "magnetic size" from Langevin fits, δM plots of all the VST series and graphical analysis of the intraparticle and interparticle contributions to the dip (Fig. S2), example of hysteresis loops measured after ZFC and FC (for sample VST17, Fig. S3); X-ray diffraction patterns and lattice parameter across of the maghemite cores of different size (Fig. S4); complete results from Monte Carlo simulations showing the dependence of δM on core anisotropy (Fig. S5), surface anisotropy ( Fig. S6), exchange coupling constant ( Fig. S7) and disordered surface thickness (Fig. S8).

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“…Experimentally accessing and quantifying the degree of aggregation, however, is by no means trivial. This problem has generated much interest lately, with recent proposals involving the use of Lorentz microscopy 63 and small angle X ray scattering. 64 The purpose of this paper is to show that AC susceptibility measurements, a technique which is easily accessible experimentally, can also yield important information concerning the state of aggregation in a sample.…”

Section: Introductionmentioning

confidence: 99%

AC susceptibility as a tool to probe the dipolar interaction in magnetic nanoparticles

Landi

Arantes

Cornejo

et al. 2017

Journal of Magnetism and Magnetic Materials

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The dipolar interaction is known to substantially affect the properties of magnetic nanoparticles. This is particularly important when the particles are kept in a fluid suspension or packed inside nanocarriers. In addition to its usual long-range nature, in these cases the dipolar interaction may also induce the formation of clusters of particles, thereby strongly modifying their magnetic anisotropies. In this paper we show how AC susceptibility may be used to obtain important information regarding the influence of the dipolar interaction in a sample. We develop a model which includes both aspects of the dipolar interaction and may be fitted directly to the susceptibility data. The usual long-range nature of the interaction is implemented using a mean-field solution, whereas the particle-particle aggregation is modeled using a distribution of anisotropy constants. The model is then applied to two samples studied at different concentrations. One consists of spherical magnetite nanoparticles dispersed in oil and the other of cubic magnetite nanoparticles embedded on PLGA nanospheres. We also introduce a simple technique to access the importance of the dipolar interaction in a given sample, based on the height of the AC susceptibility peaks for different driving frequencies. Our results help illustrate the important effect that the dipolar interaction has in most nanoparticle samples.

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Lorentz microscopy sheds light on the role of dipolar interactions in magnetic hyperthermia

Cited by 17 publications

References 47 publications

Magnetic Shape Memory Turns to Nano: Microstructure Controlled Actuation of Free‐Standing Nanodisks

Magnetic Shape Memory Turns to Nano: Microstructure Controlled Actuation of Free‐Standing Nanodisks

Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles

AC susceptibility as a tool to probe the dipolar interaction in magnetic nanoparticles

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